Method for selective deposition of a thin self-assembled monolayer

ABSTRACT

A method for selective deposition of self-assembled monolayers to the surface of a substrate for use as a diffusion barrier layer in interconnect structures is provided comprising the steps of depositing a first self-assembled monolayer to said surface, depositing a second self-assembled monolayer to the non-covered parts of said surface and subsequently heating said substrate to remove the first self-assembled monolayer. The method of selective deposition of self-assembled monolayers is applied for the use as diffusion barrier layers in a (dual) damascene structure for integrated circuits.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 11/296,033, filed Dec. 7,1995, which claims priority under 35 U.S.C. § 119(e) to U.S. provisionalapplication Ser. No. 60/634,900, filed Dec. 9, 2004, the disclosures ofwhich are hereby incorporated by reference in their entirety and arehereby made a portion of this application.

FIELD OF THE INVENTION

The present invention is related to the field of semiconductorprocessing. More specifically it is related to the field of formingself-assembled monolayers for use as diffusion barrier layers. Morespecifically it is related to field of (e.g. copper) damascene advancedinterconnect applications for integrated circuit (IC) manufacturing.

BACKGROUND OF THE INVENTION

Copper is the preferred metal for creating multilevel interconnectstructures in ultra-large-scale-integrated circuits because of its highelectrical conductivity and electromigration resistance. One of themajor challenges in Cu metallization technology is the prevention of therapid diffusion of Cu into adjoining layers of SiO₂ and relatedlow-dielectric-constant materials (e.g., fluorinated SiO_(x), SiOCH)during device operation. This is because Cu incorporation into thedielectric degrades the dielectric properties of the insulating layer,causing increased leakage currents, and leading to inferior deviceperformance and failure.

The current industry standard is 10-30-nm-thick metallic diffusionbarrier layers of Ti- or Ta-based compounds (such as Ta, TaN, TiSiN andTiN) or Cu-based alloys to alleviate this problem. While theseapproaches have been successful thus far, barriers with thicknessesbelow 5 nm will be needed at sub-100-nm feature sizes and in advancedfuture device architectures (e.g. three-dimensional integratedmultiple-wafer devices) to fully realize the advantage of Cuinterconnects. It is difficult to obtain such thin barriers withacceptable step coverage by conventional metal deposition methods(Plasma Vapor deposition (PVD) or Chemical Vapor Deposition (CVD))without compromising the barrier-layer microstructure and/or theirconformality in high-aspect-ratio features. Additionally thick diffusionbarrier layers take up the volume meant for low-resistivity Cu, reducingthe advantages of scaling (miniaturization).

Newly emerging methods such as atomic layer deposition (ALD) have thepotential to obviate some of these concerns. However, to obtainconformal films with thicknesses below 5 nm of conventional barriermaterials, it is not clear if they will be effective due to highdefectivity. Hence, there is a great deal of interest in exploringalternative materials and processing methods.

Recently, Self-Assembled Monolayers (SAM) are reported to act as thin(generally <2 nm) Cu diffusion barrier layers (US 2002/0079487). WithSAM layers comprising organo-silane molecules, Cu diffusion into thedielectric is inhibited and a good Cu adhesion at the Cu/SiO₂ interfaceis achieved. The use of said SAM layer in integrated circuit structuressuch as dual damascene structures wherein electrical contact is requiredbetween the metal line of interest to metal lines above or below thesituation is more complicated. Deposition of said SAM layer (comprisingorgano-silane molecules) to the already existing metal structure willlead to poor adhesion of Cu seed layers prior to further Cu depositionand poor electrical conductivity later on in the finalized dualdamascene structure.

SUMMARY OF THE INVENTION

The aim of the preferred embodiments is to provide a method for theselective deposition of a Self-Assembled Monolayer (SAM) by means ofapplying first a selective, releasable Self-Assembled Monolayer thatacts as a masking layer such that a second permanent Self-AssembledMonolayer can be deposited. The second permanent SAM will be used as aCu diffusion barrier deposited only on the dielectric material.

The preferred embodiments provide a method for selective deposition of athin Self Assembled Mono-layer (SAM) which acts as a copper (Cu)diffusion barrier for use with copper damascene advanced interconnectapplications for integrated circuit (IC) manufacturing.

The method for forming selective self-assembled monolayers comprises thesteps of depositing a first releasable self-assembled monolayer (SAM-1)to a copper containing surface, depositing a second permanentself-assembled monolayer (SAM-2) to the non-copper containing parts ofsaid surface and subsequently heating said substrate to remove the firstself-assembled monolayer.

The preferred embodiments are developed for improved electrical andreliability characteristics of SAM Cu diffusion barriers when applied todual damascene (DD) Cu interconnects.

The preferred embodiments solve the problem of applying a(non-selective) self-assembled monolayers for use as barrier layer indual damascene structures as described in the prior art by depositingfirst a protective (sacrificial) self-assembled monolayer (referred toas SAM-1) selectively to the copper surface of an existing metalstructure of a dual damascene structure before applying a second(permanent) self-assembled monolayer (referred to as SAM-2) to thenon-copper parts of said dual damascene structure. By applying saidsacrificial SAM-1 to the already existing copper surface of a dualdamascene structure (also referred to as underlying metal layer n−1)said existing copper surface will be protected from non-selectiveadsorption of SAM-2. The release of the sacrificial SAM-1 after SAM-2deposition on the dielectric makes it possible to create a good anddirect copper to copper contact in the final dual damascene structure orin other words a good Cu—Cu contact between the via bottom (metal n) andthe underlying metal (n−1) of a dual damascene structure can beachieved.

It is further an object of the preferred embodiments to provide amolecule suitable for use as a releasable self-assembled monolayer(SAM-1) to a copper containing surface and a product suitable for use asa second permanent self-assembled monolayer (SAM-2).

In a first aspect, a method for forming a self-assembled monolayerconfigured for use as a diffusion barrier in an interconnect structure,the method comprising the steps of providing a substrate having at leastone opening, wherein the opening comprises a bottom part and at leastone sidewall, the bottom part having a copper surface and the sidewallhaving at least one dielectric surface; selectively depositing a firstself-assembled monolayer on the copper surface; thereafter selectivelydepositing a second self-assembled monolayer on the dielectric surface;and thereafter heating the substrate to remove the first self-assembledmonolayer, wherein the remaining second self-assembled monolayer isconfigured for use as a diffusion barrier in an interconnect structure.

In an embodiment of the first aspect, the first self-assembled monolayercomprises a plurality of first molecules, wherein each first moleculecomprises a first molecule head group, a first molecule chain portion,and a first molecule terminal group, wherein the first molecule chainportion comprises a hydrocarbon, and wherein the second self-assembledmonolayer comprises a plurality of second molecules, wherein each secondmolecule comprises a second molecule head group, a second molecule chainportion, and a second molecule terminal group, wherein the secondmolecule chain portion comprises a hydrocarbon.

In an embodiment of the first aspect, the first self-assembled monolayercomprises a plurality of first molecules, each first molecule having afirst head group, a first chain portion, and a first terminal group,wherein the first head group adheres more strongly to the copper surfacethan to the dielectric surface, wherein the first terminal group adheresweakly to the copper surface and adheres weakly to the dielectricsurface, and wherein the first molecule has a low thermal stability.

In an embodiment of the first aspect, the step of heating the substrateis conducted at a temperature above 100 degrees Celsius, and wherein thefirst self-assembled monolayer has a low thermal stability at atemperature above about 100 degrees Celsius.

In an embodiment of the first aspect, the step of heating the substrateis conducted at a temperature of from about 100 degrees Celsius to about150 degrees Celsius, and wherein the step of heating has a duration ofat least about 1 minute.

In an embodiment of the first aspect, the second self-assembledmonolayer comprises a plurality of second molecules, each secondmolecule having a second head group, a second chain portion, and asecond terminal group, wherein the second head group adheres morestrongly to the dielectric surface than to the copper surface, andwherein the second terminal group adheres weakly to the dielectricsurface and strongly to the copper surface.

In an embodiment of the first aspect, the dielectric surface is aSi-based dielectric material.

In an embodiment of the first aspect, the first self-assembled monolayercomprises a plurality of molecules having a chemical formulae selectedfrom the group consisting of X—R₁—SH, X—R₁—S—S—R₂—Y, R₁—S—R₂, andcombinations thereof, wherein R₁ and R₂ are independently a carbon chainor a carbon chain interrupted by at least one heteroatom. X and Y can bechemical groups that essentially do not chemically react with the coppersurface. At least one of R₁ and R₂ can be a chain of n carbon atoms,wherein n is an integer of preferably from 1 to 30, more preferably from1 to 18, or most preferably from 6 to 16. At least one of R₁ and R₂ canbe a chain of n carbon atoms interrupted by p heteroatoms, wherein n+pis an integer of preferably from 1 to 30, more preferably from 1 to 18,or most preferably from 6 to 16. The carbon chain can comprise a portionselected from the group consisting of alkyl, alkenyl, alkynyl, cyclicalkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl, alkynyl boundto aryl, and combinations thereof X can be methyl or hydrogen. The firstself-assembled monolayer can have a chemical formula SH(CH₂)₉CH₃.

In an embodiment of the first aspect, the second self-assembledmonolayer has a chemical formula (Z)₃SiR₃SH, wherein R₃ is a carbonchain or a carbon chain interrupted by at least one heteroatom. Z can beindependently selected from the group consisting of CH₃, Cl, C₂H₅, OCH₃,and OC₂H₅. R₃ can be a chain of n carbon atoms, wherein n is an integerof preferably from 1 to 30, more preferably from 1 to 18, or mostpreferably from 6 to 16. R₃ can be a chain of n carbon atoms interruptedby p heteroatoms, wherein n+p is an integer of preferably from 1 to 30,more preferably from 1 to 18, or most preferably from 6 to 16. Thesecond self-assembling monolayer can have a chemical formula SH(CH₂)₁₀SiCl₃.

In an embodiment of the first aspect, the interconnect structure is adamascene structure. The damascene structure can be a dual damascenestructure.

In a second aspect, a diffusion barrier layer in an interconnectstructure is provided, wherein the diffusion barrier is a self assembledmonolayer comprising a plurality of molecules of chemical formulaSH(CH₂)₁₀SiCl₃.

In a third aspect, a releasable self-assembled monolayer is provided,the monolayer comprising a plurality of molecules of chemical formulaSH(CH₂)₉CH₃, wherein the monolayer is configured to act as a protectinglayer for a copper surface in an interconnect structure.

In a fourth aspect, a semiconductor device is provided, the devicecomprising a self-assembled monolayer selectively deposited onto atleast one dielectric part of a damascene structure.

In an embodiment of the fourth aspect, the damascene structure is a dualdamascene structure.

In an embodiment of the fourth aspect, the self-assembled monolayercomprises a plurality of molecules of chemical formula SH(CH₂)₁₀SiCl₃.

In an embodiment of the fourth aspect, the semiconductor device furthercomprises a direct copper-to-copper contact between copper at a bottomof a via and copper in a trench of an underlying level.

BRIEF DESCRIPTION OF THE DRAWINGS

All drawings are intended to illustrate some aspects and embodiments ofthe present invention. Devices are depicted in a simplified way forreason of clarity. Not all alternatives and options are shown andtherefore the invention is not limited to the content of the givendrawings. Like numerals are employed to reference like parts in thedifferent figures.

FIG. 1 shows a schematic representation of a SAM molecule consisting ofa terminal functional group 1, a hydrocarbon chain 2 and a head group 3.

FIGS. 2A-2D show dual damascene integration structures before and afterSAM integration as described in prior art. FIG. 2A shows a dualdamascene starting structure before SAM deposition. FIG. 2B shows a dualdamascene structure with a uniform SAM deposition (prior art). FIG. 2Cshows a dual damascene structure after seedlayer deposition onto SAMdeposited surfaces. FIG. 2D shows the final dual damascene structureafter seedlayer deposition, copper plating and subsequent planarization.

FIGS. 3A-3F show dual damascene integration structures before and afterselective SAM integration as described in the preferred embodiments.FIG. 3A shows a dual damascene starting structure before SAM deposition.FIG. 3B shows a dual damascene structure with a depositing a sacrificialSAM-1 onto the existing Cu surface.

FIG. 3C shows a dual damascene structure after depositing a permanentSAM-2 onto the dielectric. FIG. 3D shows the release of the sacrificialSAM-1 by thermal anneal. FIG. 3E shows seedlayer deposition onto SAM-2coated dielectric and Cu surface. FIG. 3F shows the final dual damascenestructure after seedlayer deposition, copper plating and subsequentplanarization.

FIG. 4 illustrates cyclic voltammograms collected with a scan rate 50mVs⁻¹ in aqueous solutions of 0.1 M NaOH showing the influence of SAM-1(1-decanethiol, C10) on Cu oxidation and reduction. Films of SAM-1 wereformed by immersion (>2 hours) in 10⁻³ M solutions in isopropanol.

FIG. 5 illustrates cyclic voltammograms collected with a scan rate 50mVs⁻¹ in aqueous solutions of 0.1 M NaOH showing the influence of SAM-1and SAM-2 on Cu oxidation and reduction. Films were formed by immersionof Cu for 1 hour in a 10⁻³ M 1-decanethiol or MPTMS solution inisopropanol and toluene, respectively.

FIG. 6 shows the sheet resistance (Rs) versus annealing temperatureplots for Cu/SAM/SiO₂ where the SAM molecule is CN(CH₂)₁₁SiCl₃ (CN-SAM)or HS(CH₂)₃Si(OCH₃)₃ (SH-SAM). A reference Cu/SiO₂ sample is shown forcomparison.

FIG. 7 shows the sheet resistance (Rs) versus annealing temperatureplots for Cu/SAM/SiO₂ where the SAM molecule is CH₃(CH₂)₇SiCl₃,CH₃(CH₂)₁₀SiCl₃, CH₃(CH₂)₁₁SiCl₃, CH₃(CH₂)₁₇SiCl₃, Br(CH₂)₁₁SiCl₃ orHS(CH₂)₃Si(OCH₃)₃. A reference Cu/SiO₂ sample is shown for comparison.

FIG. 8 shows the sheet resistance (Rs) versus annealing temperatureplots for Cu/SAM/SiO₂ where the SAM molecule is CH₃(CH₂)₇SiCl₃,CH₃(CH₂)₁₀SiCl₃, CH₃(CH₂)₁₁SiCl₃, CH₃(CH₂)₁₇SiCl₃, Br(CH₂)₁₁SiCl₃ orHS(CH₂)₃Si(OCH₃)₃. (similar to FIG. 7 without showing the referenceCu/SiO₂ sample).

FIG. 9 illustrates the Thermal Desorption Spectra (TDS) forCH₃(CH₂)₉S—Cu (or in other words CH₃(CH₂)₉SH bonded to a Cu-surface). Itclearly shows a desorption maximum for CH₃—(CH₂)₉—SH bonded to copper atless than 100 degrees Celsius.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples illustrate a preferred embodimentin detail, including molecules, the use of said molecules and a methodfor forming a selective self-assembled monolayer (SAM) and itsapplications. It will be appreciated that there are numerous variationsand modifications possible. Accordingly, the description should not bedeemed to limit the scope of the invention, which is defined by theclaims.

SAM Selection for Selective Cu Diffusion Barrier Implementation

The key physical attributes needed for successful integration of a newbarrier for Cu dual damascene (DD) are good adhesion, good thermalstability, low defectivity and robust electrical connection between thetop (n) and bottom (n−1) metals. An added benefit to the integration ofa selective SAM deposition process is the intimate Cu—Cu contact betweenthe via bottom (metal n) and the underlying metal (n−1). The eliminationof the refractory metal diffusion layer between Cu layers (n & n−1)represents a potential electro-migration (EM) benefit in that the fluxdivergence at the via is eliminated and the intrinsic Cu EM can berealized.

For the purpose of the preferred embodiments, damascene processingshould be understood as a dielectric etch approach for fabricating(copper) interconnect structures. A wide variety of applications (e.g.,DRAM, ASIC, MPU, and SOC) involve the damascene sequence. Damascenerefers to the process by which a metal conductor pattern is embeddedwithin a non-conducting (dielectric) material. The process of creatingsingle damascene (SD) structures comprises first etching a trench orgroove in a planarized dielectric layer followed by filling said trenchwith a metal such as copper (further involving planarization of themetal towards the dielectric). In dual damascene (DD) the processsequentially creates embedded vias and trenches, with the vias formingthe vertical connections from one layer of circuitry (trenches) to thenext. Or in other words, in a dual damascene structure a second level ofdielectric is involved in which a series of holes (i.e. vias orcontacts) are etched and filled with metal, said holes making contact tothe trenches in the dielectric level underneath.

For the purpose of the preferred embodiments, self-assembled monolayers(SAM) should be understood as a relatively ordered assembly of moleculesthat spontaneously adsorb (also called chemisorb) from either the vapouror liquid phases on a surface. In general the SAM film is engineered bythe selection of the head group, the hydrocarbon comprising chain andterminal group of the molecule as defined in FIG. 1. The head 3, theterminal group 1 and hydrocarbon chain length 2 can be selectedindependently. In the description below, the head group 3 is defined asthe end of the SAM molecule that is compatible with and bonds well tothe substrate, preferably here Cu or SiO₂. The terminal group 1 isdefined as the end that is exposed at the SAM-gas/liquid interface. Thechain length of the hydrocarbon 2 influences the order and packingdensity of the SAM.

It is an object of the preferred embodiments to provide a method forselectively depositing a SAM barrier layer. To realize the finalselective deposition of a SAM barrier, preferably two SAM films areused. Preferably a first SAM layer, referred to as SAM-1, is appliedfirst as a releasable and temporary masking film and is selectivelydeposited on the metal surface but not on the dielectric. The metal ispreferably copper. A second SAM layer, referred to as SAM-2, is thenselectively deposited on the dielectric. SAM-2 is a permanent (diffusionbarrier) layer. The masking film (SAM-1) will protect the copper surfacefrom damage (such as oxidation and reduction reactions occurring at thecopper surface of the underlying metal (n−1) at the via bottom) duringSAM-2 deposition. Furthermore it will protect the copper surface fromSAM-2 deposition. After the deposition of SAM-2, SAM-1 is preferablyreleased from the copper surface, most preferred said release process isa low temperature anneal (less than 150 degrees Celsius).

The selective nature of the SAM barrier depositions and release, asdescribed here is the result of the selection of the terminal and headgroups of the SAM molecules. Preferably the head group of SAM-1 hasstrong adhesion (bonding) to the metal surfaces and weak adhesion todielectric surfaces. The terminal group of SAM-1 has weak adhesion tothe metal and dielectric surfaces. These adhesion characteristics resultin the selective deposition where the head group is attached to themetal with the terminal group exposed at the gas/liquid-SAM interfaceand there is no deposition on the dielectric surface. SAM-1 is furtherselected for its low thermal stability. The terminal group of SAM-2 hasstrong adhesion (bonding) to metal surfaces and weak adhesion todielectric and SAM-1 coated surfaces.

In a preferred embodiment, SAM-1 is characterized as comprising at leastone molecule with the chemical formula: X—R₁—SH or X—R₁—S—S—R₂—Y orR₁—S—R₂ wherein R₁ and R₂ are hydrocarbon segments comprising chains ofn carbon atoms, optionally interrupted by heteroatoms. X—R₁—S—S—R₂—Ybased molecules will break into two R—S subunits forming a SAM-1molecule identical to X—R1-SH. R₁ and R₂ are independent of each otherand can have the same chemical formula or can be different. X and Y canhave the same chemical formula or can be different. X and Y are chemicalgroups selected such that there is essentially no chemical reactionbetween X and the metal (Cu) or dielectric surfaces. This means thatthere is no competition between the sulphur atom and X related tochemisorption on the metal surface.

R₁ or R₂ used in SAM-1 is a chain of n carbon atoms, optionallyinterrupted by p heteroatoms wherein n or (n+p) is an integer between 1and 30, more preferred between 1 and 18, and most preferred between 6and 16. Said (hydro)carbon chain promotes the formation of aself-assembling monolayer. Said carbon chain may be understood asincluding an alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound toaryl, alkenyl bound to aryl, alkynyl bound to aryl. Said (hydro)carbonchain can be branched. All combinations of R₁, R₂, the number of carbonatoms and the interruption by heteroatoms are possible.

In a most preferred embodiment, the molecule forming the SAM-1 layer isX—R₁—SH, R₁ being an alkyl chain —(CH₂)_(n)—. Preferably n is between 1and 30, more preferred between 1 and 18 and most preferred between 6 and16. Most preferred the X group is a methyl (CH₃) or hydrogen (H) group.An example of said composition is SH(CH₂)₉CH₃ (also referred to asC₁₀H₂₁SH, decylmercaptan, 1-decanethiol or 1-mercaptodecane).

In an alternative embodiment, the molecule forming the SAM-1 is X—R₁—SH,R₁ comprising an alkyl group and an aryl group. X₁, X₂, X₃, X₄, X₅ andX₆ are preferably H, CH₃ or (CH₂)_(n).

In Another alternative embodiment, the molecule forming the SAM isX—R₁—SH, R₁ comprising an aryl group. X₁, X₂, X₃, X₄, X₅ and X₆ arepreferably H, CH₃ or (CH₂)_(n).

Preferably SAM-2 is characterized as comprising at least one moleculewith the chemical formula: (Z)₃Si—R₃—SH wherein R₃ is a carboncomprising chain of carbon atoms (optionally interrupted byheteroatoms), (Z)₃Si is the head group and the SH group is the terminalgroup of the SAM-2 molecule. The length of the carbon comprising chainR₃ is such that it is long enough to obtain a good packing density(leading to a good surface coverage of the dielectric) but still keepingthe SAM-2 molecule in a liquid state at room temperature. The carbonchain is preferably an alkyl chain which is formed by alkanes. Thisalkyl chains will lead to Van der Waals interactions obtaining a closepacked order in the SAM-2 monolayer. The Z component in the head groupis preferably a CH₃, Cl, C₂H₅, OCH₃, OC₂H₅ group. All combinations ofthese groups are possible to define Z. Examples of possible head groupsare SiCl₃, Si(OCH₃)₃, SiCl(OCH₃)₂, etc. The head group is chosen suchthat there is a good binding or strong adhesion to the dielectriccombined with a good surface coverage of the dielectric. The head groupof SAM-2 is further characterized as having a weak adhesion to copperand SAM-1 coated surfaces. The terminal group (SH is most preferred butother examples are possible) of SAM-2 must have strong adhesion(bonding) to metal surfaces and weak adhesion to dielectric and SAM-1coated surfaces.

In a preferred embodiment, SAM-2 is characterized as comprising at leastone molecule with the chemical formula: (Z)₃Si—(CH₂)_(n)—SH wherein n ispreferably between 1 and 30, more preferred between 1 and 18 and mostpreferred between 6 and 16. Z is preferably a CH₃, Cl, C₂H₅, OCH₃, OC₂H₅group. All combinations of these groups are possible to define Z.Examples of possible head groups are SiCl₃, Si(OCH₃)₃, SiCl(OCH₃)₂, etc.An example of a preferred SAM-2 molecule has following structure:SH(CH₂)₁₀SiCl₃.

In a preferred embodiment the dielectric surface is a Si baseddielectric. Examples of these Si based dielectric materials, but notlimited hereto, are materials such as SiO₂, SiC, SiCO(H), SiC(N),including Silicon based low-k films. Surface treatments of low-k filmsto make the surface more hydrophilic may be necessary in some cases. Themetal surface is preferably a copper containing surface and is part of acopper interconnect structure such as a via or trench in a dualdamascene structure.

Preferably the principle of selectively depositing a thin Self AssembledMono-layer (SAM) will be used for selective deposition of a copperdiffusion barrier in dual damascene interconnect structures. Theprinciple of selectively depositing a thin Self Assembled Mono-layer(SAM) can also be applied as diffusion barriers for other conductivematerials and in future technologies.

Selective Self-Assembled Monolayers for Use in Copper (dual) DamasceneStructures

The method of the preferred embodiments for selectively depositing a SAMdiffusion barrier layer is applied for diffusion barrier deposition indamascene structures, preferably in dual damascene structures.

FIGS. 2A to 2D show schematic diagrams of a Dual Damascene (DD)structure prior to and after the SAM and Cu seed depositions asdescribed in the prior art. In FIG. 2B, a uniform SAM deposition 5 isassumed on all surfaces. The SAM molecule as described in the prior artcorresponds to SH(CH₂)₃Si(OCH₃)₃. The final obtained integratedstructure as shown in FIG. 2D (after copper seed deposition and Cuplating) is expected to have marginal or poor SAM-seed adhesion 6 overthe via structure and questionable electrical connection between theupper Cu seed layer 7 and lower metal 4. In FIGS. 3A to 3F, using themethod of the preferred embodiments, a selective SAM-2 8 deposition isassumed to dielectric surfaces 11 only. In this case there will be noSAM barrier between the upper and lower metal (indicated as 9 in FIG.3F). The intimate contact of the top and bottom Cu provides goodelectrical contact and eliminates the Cu flux divergence at the lowermetal-via interface, which is present in refractory metal barrier cases.The absence of the Cu flux divergence is expected to have positiveeffect on the EM resistance and potentially eliminate

Stress Induced Voiding (SIV) at the Bottom of the Via.

Using FIGS. 3A to 3F, the preferred method can be summarized as follows.FIG. 3A shows a dual damascene starting structure before SAM deposition.As shown in FIG. 3A, the method starts from a substrate, said substratehaving a first dielectric layer 10 having a first copper structure 4(also referred to as metal n−1) embedded in said first dielectric layer10. Onto said first dielectric layer 10, a second dielectric layer 11 isdeposited and a dual damascene structure is patterned into said seconddielectric 11 layer e.g. using a via first approach. FIG. 3B shows thedual damascene structure after depositing a sacrificial SAM-1 13 ontothe existing Cu surface 4. FIG. 3C shows the dual damascene structureafter depositing a permanent SAM-2 8 only onto the dielectric surfaces11. FIG. 3D shows the release of the sacrificial SAM-1 13 by thermalanneal. Said thermal anneal is preferably at temperatures at about 100to 150 degrees Celsius, or in other words preferably above 100 degreesCelsius (e.g. preferred temperature of 150 degrees Celsius). The maximumanneal temperature is defined by the thermal stability of the secondself-assembled monolayer (SAM-2). In the preferred embodiments thechemical structure of SAM-2 is such that it has preferably a thermalstability above 500 degrees Celsius (no desorption of SAM-2 attemperatures below 500 degrees Celsius). The thermal anneal of SAM-1 isfurther characterized as a rapid thermal anneal (e.g. 0.4 degrees persecond) which is performed within a few minutes (e.g. 5 minutes). FIG.3E shows seedlayer deposition 7 onto SAM-2 coated dielectric surfaces 8and Cu surface 4. FIG. 3F shows the final dual damascene structure afterseedlayer deposition 7, copper plating 12 (resulting in metal n) andsubsequent planarization.

EXAMPLES Example 1 SAM Preparation and Coating onto Wafer Substrates byLiquid Immersion

SAM-1 was prepared by immersion of Cu plated wafer substrates in adilute solution of 1-decanethiol. The dilute solution was obtained bydilution of 1 ml of concentrated 1-decanethiol in 5000 ml solvent, usingisopropyl alcohol (IPA) as solvent at ambient temperature for 2 h. TheSAM-1 chemical structure is CH₃—(CH₂)₉—SH, also referred to as C10 or1-decanethiol. The SAM-1 material has 96% purity and is obtained fromSigma-Aldrich. The C10-modified wafer was then rinsed with copiousamounts of IPA (30 sec) and dried under nitrogen. The substrates werethen electroplated with copper. The Cu substrates were cleaned byimmersion in 3.7% hydrochloric acid for 5 min. followed by rinsing withcopious amounts of deionised water (2 min) and then solvent (30 sec)immediately before immersion in thiol solution.

SAM-2 was prepared by rinsing SiO₂ substrate in toluene (10 sec) andthen immersion in a dilute solution of mercaptopropyltrimethoxy-silane.The dilute solution was obtained by dilution of 10 ml of concentratedmercaptopropyltrimethoxy-silane in 5000 ml solvent, using toluene assolvent at ambient temperature for 1 h. Concentratedmercaptopropyltrimethoxy-silane (also referred to as MPTMS) was used asreceived from Gelest. The MPTMS-modified wafer was then rinsed withcopious amounts of toluene (30 sec), then, acetone (30 sec) and,finally, ethanol (30 sec) before being dried under nitrogen flow.

In order to verify that SAM-1 does not adsorb on SiO₂, SiO₂ substrateswere also exposed to SAM-1 under the same conditions as described forCu. To investigate the adsorption of SAM-2 on Cu, Cu substrates wereexposed to SAM-2 under the same conditions as described for SiO₂.Further, SAM-1 films on Cu and SAM-2 films on SiO₂ were exposed to SAM-2and SAM-1, respectively.

Example 2 SAM Preparation and Coating onto Wafer Substrates by VapourDeposition

SAM-1 or SAM-2 can also be adsorbed from the vapour phase. SAM-1 orSAM-2 was contained in a pyrex glass tube and connected via a gas lineto a high vacuum system containing the substrate of interest. There wasa valve between the vacuum system and the glass tube. By opening thisvalve, the substrate contained in the vacuum system was exposed to thevapour of SAM-1 or SAM-2 for different times depending on the dosingpressure.

Example 3 Contact Angle (CA) Measurements

Water CA measurements were used to assess SAM quality, more specificallyadhesion properties to the different substrates and thermal stability ofthe SAM layers. Static contact angles of deionized water deposited onthe samples were measured in air using a software-controlled VideoContact Angle System OCA-20 (DataPhysics). All angles measured aresubject to an error of ±0.1°. In addition, a variation of ±2.5° wastypically observed across a given blanket Cu wafer. A minimum of 6measurements/sample were performed for all samples.

From the contact angle measurements it was concluded that SAM-1 isdeposited on Cu plated substrates (D07) but not on SiO₂ substrates(D09). SAM-2 is deposited on the SiO₂ substrates (D08)

SAM-1 on Cu substrates is released (D07, D07PA) by the anneal, 250° C.for 10 min in an N₂ ambient. SAM-2 on SiO₂ is not affected (D08, D08PA)by the anneal and thus thermally stable.

Sequential treatments of SAM-1 and SAM-2, required for the selective SAMdeposition, were also investigated. SAM-½ on Cu (D05) has the same CA asSAM-1 on Cu (D07) indicating that SAM-2 is not deposited on SAM-1. SAM-½on SiO₂ (D18) has the same CA as SAM-2 on SiO₂ (D08) consistent with theprior result that SAM-1 does not deposit on SiO₂. SAM-½ on Cu afteranneal (D15PA) has the same CA as post anneal SAM-1 on Cu (D07PA)indicating that the all material on the Cu is release after the 2 stepprocess. SAM-½ on SiO₂ after anneal (D18PA) has the same CA as postanneal SAM-2 on SiO2 (D08PA) indicating that the 2 step process isthermally stable like the SAM-2 on SiO₂ case. TABLE 1 Contact Angle dataPhase Anal Sub SAM Anneal Wafer 1 2 Meth Cu 1 No D07 100 126 CA Cu 1 YesD07 PA 73 50 CA Cu 2 No D03 57 56 CA Cu 2 Yes D03 PA 62 41 CA Cu 1/2 NoD05 92 116 CA Cu 1/2 Yes D05 PA 60 62 CA SiO2 1 No D09 8 5 CA Yes D09 PA22 CA SiO2 2 No D08 53 57 CA SiO2 2 Yes D08 PA 54 59 CA SiO2 1/2 No D1845 52 CA SiO2 1/2 Yes D18 PA 39 53 CA Cu Ref None No D06 59 58 CA YesD06 PA 15 36 CA SiO2 Ref None No D12 11 10 CA Yes D12 PA 12 43 CAPA = Post Anneal

Example 3 Adhesion Measurements

Adhesion measurements were made on 4 wafers (indicated as D13, D14, D15and D16 in Table 2) that continued processing (Cu seed deposition) afterthe SAM deposition. The adhesion measurements were performed by cleaving2 sections (cords) from the wafer, applying tape (Scotch “red” crystalclear tape) to the pieces and pulling the tape from the cleaved edge ofthe wafer. The tape is pulled with a steady motion parallel to the wafersurface. Several (5-6) pieces of tape are attached to the wafer andpulled sequentially. The test was performed on 2 pieces per wafer to getsufficient sample size. Each tape pull is considered a trial. Failure isdefined as a test which results in Cu on the tape. The yield iscalculated by #_pass/#_trials (%). The results of the adhesion testshowed good adhesion properties for SiO₂/SAM-2/Cu (D16) and a pooradhesion for Cu/SAM-1/Cu & Cu/SAM-1/SAM-2/Cu (D13 & D15). TABLE 2Adhesion test results Adhesion wafer substrate SAM Pass trials yield D13Cu 1 0 11  0% D14 Cu 2 11 11 100% D15 Cu 1 + 2 0 10  0% D16 SiO2 2 10 10100%

Example 4 Proof of Concept for Selective Process

Cyclic voltammetry was used to evaluate SAM-1 stability (afterdepositing said SAM-1 molecule on a Cu surface) againstelectrochemically induced oxidation-reduction corrosion. FIG. 4 showscurrent-potential cyclic curves in 0.1 M aqueous NaOH solution with apositive sweep from −1.05 to 0.05 V (versus Ag/AgCl) at a scan rate of0.05 Vs⁻¹. For clean, non-passivated Cu (or in other words a coppersurface without SAM-1 deposition), the anodic and cathodic peaks andtheir assignments are indicated. The corresponding current-potentialcyclic curve for Cu passivated with SAM-1 (1-decanethiol, also referredto as Decylmercaptan, C₁₀H₂₁SH or “C10”) is also shown in FIG. 4. Thepassivation was optimised by forming films of SAM-1 on the Cu-surface byimmersion for a minimum of 2 hours in 10⁻³ M 1-decanethiol solution inisopropanol) to completely quench Cu oxidation-reduction processes.

In FIG. 5, the current-potential cyclic curves for Cu passivated withSAM-2 show that this molecule also forms a layer on Cu that is partiallypassivating under the film formation conditions employed that arecomparable with those used to form a SAM-2 film on SiO₂ (immersion for 1hour in a 10⁻³ M Mercaptopropyltrimethoxysilane (MPTMS) solution intoluene). Clearly, the passivation process was not optimum, i.e., the Cuoxidation-reduction processes are suppressed but not completelyquenched. Nevertheless, this experiment confirms that SAM-2 does form afilm on Cu, in agreement with literature. Hence this result shows theneed to protect the Cu-surface (in a dual damascene structure) duringSAM-2 processing of Cu—SiO₂ structures.

Example 5 Screening of SAMs as Potential Cu Diffusion Barriers

As a rough evaluation of SAM barrier properties against Cu diffusion weannealed Cu/SAM/SiO₂ samples to different temperatures (200, 250, 290,340, 380, 420, 460 and 500° C.) and then measured the sheet resistance(Rs). A high increase in sheet resistance (Rs) indicates that Cu₃Si isformed and implies that there is failure of the barrier for copperdiffusion inhibition (via openings or pin holes in the barrier). TheSAM-2 molecules were evaluated in terms of different alkyl chainlengths, head groups and terminal groups as listed in Table 3. Theresulting Rs versus annealing temperature plots are shown in FIGS. 6, 7and 8. TABLE 3 Name, chemical formula, abbreviation, and water contactangle (CA) measured for as-prepared SAMs formed on SiO₂. Chemical NameFormula Abbreviation CA° Octyltrichlor- CH₃(CH₂)₇SiCl₃ CH₃—C₇-SAM 111.5osilane undecyltri- CH₃(CH₂)₁₀SiCl₃ CH₃—C₁₀-SAM 113.0 chlorosilanedodecyltri- CH₃(CH₂)₁₁SiCl₃ CH₃—C₁₁-SAM 112.8 chlorosilane octadecyltri-CH₃(CH₂)₁₇SiCl₃ CH₃—C₁₇-SAM 110.8 chlorosilane 11-bromoundecyl-Br(CH₂)₁₁SiCl₃ Br-SAM 86.6 trichlorosilane 11-cyanoundecyl-CN(CH₂)₁₁SiCl₃ CN-SAM 77.6 trichlorosilane 3-mercaptopropyl-HS(CH₂)₃Si(OCH₃)₃ SH-SAM 54 trimethoxysilane

FIG. 6 shows a comparison between CN-SAM and SH-SAM and a referenceCu/SiO₂ sample. In these cases, Cu diffuses through the SAM-2 and thinoxide layer to form Cu silicide upon annealing above 200° C. Clearly,the presence of SAM-SH is similar to the reference sample. In contrast,CN-SAM shows evidence of inhibiting Cu diffusion. The poor diffusioninhibition of the SH-SAM molecule is due to a too short alkyl chain,which leads to pins holes, the CN-SAM molecule shows good diffusioninhibiting properties. The terminal group “SH” of the SH-SAM is howeverpreferred to have good copper to copper bonding.

FIGS. 7 and 8 show a comparison between various SAM-2 molecules and areference Cu/SiO₂ sample. In this case, the experiments were performedon wafer pieces attached to a wafer and hence there is a temperatureoffset compared with the experiment performed on full wafers plotted inFIG. 6. From FIGS. 7 and 8, it is observed that Cu could diffuse throughthe SAM-2 and thin oxide layer to form Cu silicide upon annealing above250° C. The resulting Rs depends strongly on the SAM employed. There isevidence of a trend. The longer the alkyl chain, the better the barrierproperties. Also, adding a reactive terminal group such as Br instead ofthe unreactive methyl group enhances barrier properties. The very shortchain SH-SAM shows the poorest barrier performance but is still betterthan Cu/SiO₂.

Example 6 Desorption of SAM-1 from Cu-Surface: Investigation by ThermalDesorption Spectroscopy (TDS)

Thermal Desorption Spectroscopy was used to investigate the thermaldesorption of SAM-1 from a Cu surface. Thermal Desorption Spectroscopy(TDS), also named Temperature Programmed Desorption (TPD) is arelatively simple technique in surface science for evaluating desorptionof compounds from a surface.

A substrate, in this example a wafer surface, is plated with copper.Said substrate with copper on top of it, is immersed in a 10⁻¹ MCH₃(CH₂)₉SH(SAM-1) solution (in alcohol) for 2 hours and subsequentlyrinsed in alcohol (dried in nitrogen flow).

In the TDS experiment to investigate the SAM-1 desorption from a Cusurface, we heat the SAM-1 covered Cu-surface sample with a definedheating rate Δ=dT/dt, from a temperature below to a temperature abovethe expected desorption temperature of the adsorbate (SAM-1), andsimultaneously detect the desorbing species with a mass spectrometer.

For SAM-1 desorption studies the temperature was ramped up by 0.4degrees per second (=rapid) during the TDS analysis. FIG. 9 illustratesthe TDS spectra for CH₃(CH₂)₉S—Cu (or in other words CH₃(CH₂)₉SH bondedto a Cu-surface). It clearly shows a desorption maximum for CH₃(CH₂)₉SHbonded to copper at 100 degrees Celsius.

During processing it is therefore desired to have a thermal anneal whichis rapidly ramped up to 150 degrees Celsius and which is held at 150degrees for approximately 5 minutes.

The thermal release (or thermal stability) for the preferredSH(CH₂)₁₀SiCl₃ (SAM-2) molecule is above 500 degrees Celsius.

All references cited herein are incorporated herein by reference intheir entirety. To the extent publications and patents or patentapplications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention as embodied in the attached claims.

1. A diffusion barrier layer in an interconnect structure, wherein the diffusion barrier is a self assembled monolayer comprising a plurality of molecules of chemical formula SH(CH₂)₁₀SiCl₃.
 2. A releasable self-assembled monolayer, the monolayer comprising a plurality of molecules of chemical formula SH(CH₂)₉CH₃, wherein the monolayer is configured to act as a protecting layer for a copper surface in an interconnect structure.
 3. A semiconductor device, the device comprising a self-assembled monolayer selectively deposited onto at least one dielectric part of a damascene structure.
 4. The semiconductor device of claim 3, wherein the damascene structure is a dual damascene structure.
 5. The semiconductor device of claim 3, wherein said self-assembled monolayer comprises a plurality of molecules of chemical formula SH(CH₂)₁₀SiCl₃.
 6. The semiconductor device of claim 3, further comprising a direct copper-to-copper contact between copper at a bottom of a via and copper in a trench of an underlying level. 